The use of spherical exhaust nozzle in high performance gas turbine engine powered aircraft is known. Such nozzles are provided with movable elements to selectably control nozzle area and also change the direction of the engine exhaust gases. Such an exhaust nozzle is shown in U.S. Pat. No. 4,836,451.
In current designs of spherical exhaust nozzles, cooling air flows between the fixed spherical structure and the movable convergent flaps before entering the upstream end of the flap. Since the flaps are mounted on a gimbal to afford thrust vectoring as well as nozzle area control, construction requires a complicated seal system to contain the cooling air. Also, when the nozzle throat area increases from dry power to maximum afterburning, the volume of the gas passage increases, temporarily cutting off cooling airflow to the convergent and the divergent flaps. In addition, the convergent flaps are actuated through concentric shafts located at their pivots. This induces a large torque load on the smallest part of the convergent flap structure.